Earlier drafts of this article were presented to audiences at Newcastle University (February 2009), Oxford University (May 2009), Yale University (September 2009), McGill University (September 2009), University College London (October 2009), Manchester University (February 2010) and Princeton University (April 2010). Particular thanks for their comments are due to Robert Audi, Charles Beitz, Richard Bellamy, Alex Brown, Basak Cali, Stephen de Wijze, John Horton, Peter Jones, George Kateb, George Letsas, Jacob Levy, Andy Mason, Tariq Modood, Bob Morris, Avia Pasternak, Alan Patten, Bhikhu Parekh, Philip Pettit, Jonathan Quong, Michael Ridge, Hillel Steiner, Laura Valentini and Daniel Weinstock. Bob Goodin and two anonymous readers for the Journal of Political Philosophy also helped me with the final version. I am grateful to the Arts and Humanities Research Council (AHRC) for awarding me a Research Leave Fellowship, during which the research for this article was completed.